Photovoltaic cells of III-V compound semiconductor materials have been provided in recent years. Compared to the silicon-based photovoltaic cell, the photovoltaic cells made of the III-V compound semiconductor have greater energy conversion efficiency and higher radiation resistance. The conversion efficiency of the III-V compound semiconductor photovoltaic cell is higher than that of the silicon-based photovoltaic cell because the III-V compound semiconductor photovoltaic cell splits the spectrum of the incident radiation by multiple subcells having different energy band gaps, and accumulates the photon induced current from each subcell.
FIGS. 1A-1D illustrate a manufacturing method of a conventional multi-junction photovoltaic cell 1. As shown in FIG. 1A, an InGaP-based subcell 12 having energy gap of 1.8 eV, a GaAs-based subcell 14 having energy gap of 1.4 eV and an InGaAs-based subcell 18 having energy gap of 0.8 eV are sequentially grown on a GaAs substrate 10. Because the InGaP-based subcell 12 is lattice matched to the GaAs substrate 10, the InGaP-based subcell 12 is grown on the GaAs substrate 10 first. The GaAs-based subcell 14 and the InGaAs-based subcell 18 are then sequentially grown on the InGaP-based subcell 12. The InGaAs-based subcell 18 is substantially lattice mismatched to the GaAs-based subcell 14 and the GaAs substrate 10. A buffer layer 16 made of III-V semiconductor material is formed between the GaAs-based subcell 14 and the InGaAs-based subcell 18. Thus, the InGaP-based subcell 12, the GaAs-based subcell 14 and the InGaAs-based subcell 18 are stacked on the GaAs substrate 10 in the order from the highest energy gap to the lowest energy gap.
As shown in FIG. 1B, a silicon substrate 11 is further formed on a side (not shown) of the InGaAs-based subcell 18 opposite to the GaAs substrate 10, and then the GaAs substrate 10 is removed. As shown in FIG. 1C, the InGaP-based subcell 12, the GaAs-based subcell 14, the InGaAs-based subcell 18 and the silicon substrate 11 are flipped such that the silicon substrate 11 is provided on a bottom side (not shown) of the photovoltaic cell 1. The InGaP-based subcell 12, the GaAs-based subcell 14 and the InGaAs-based subcell 18 are provided from the highest energy gap to the lowest energy gap to receiving the different regions of the solar spectrum. The InGaP-based subcell 12 having the highest energy gap is provided on a top side (not shown) of the photovoltaic cell 1 to be a top subcell of the photovoltaic cell 1 facing the solar radiation.
As shown in FIG. 1D, a top electrode 15 is formed on the InGaP-based subcell 12 and a bottom electrode 13 is formed on the silicon substrate 11. The top electrode 15 and the bottom electrode 13 collect the photon induced current generated from each subcell 12, 14 and 18.